Formability is the ability of a given metal blank to undergo plastic deformation without being damaged. The plastic deformation capacity of metallic materials, however, is limited to a certain extent, at which point, the material could experience tearing or fracture (breakage). One main failure mode is caused by tearing of the material. This is typical for sheet-forming applications. A neck may appear at a certain forming stage. This is an indication of localized plastic deformation. Whereas homogeneous deformation generally takes place in and around the subsequent neck location in the early stable deformation stage, almost all deformation is concentrated in the neck zone during the quasi-stable and instable deformation phase. This leads to material failure manifested by tearing.
From the metallurgical perspective, the formability of a particular metal depends on the metal's elongation, which is the total amount of strain measured during tensile testing. A metal with a large elongation has good formability because the metal is able to undergo a large amount of strain (work) hardening. Strain hardening results in an increase of the load-carrying capacity of a metal as it deforms. It also prevents strains from being localized during forming, so the deformation is uniformly distributed throughout a particular section of the material that is exposed to a specific set of forming stresses. Thus, each localized region of the metal thins uniformly during the forming process.
The load carrying capacity of the metal as it deforms is opposed by the reduction in cross-sectional area of the metal as it thins. There is a maximum load where the increase in stress due to the decrease in the metal cross-sectional area becomes greater than the increase in the load-carrying ability of the metal due to strain hardening. Necking begins at this point as the metal starts to thin more in a localized region. Any additional deformation is generally concentrated in the necking area, while the loads in the surrounding areas decrease.
Determining the mechanical properties is one of the key issues in analyzing the deformation behavior of the sheet metal materials. The uniaxial tensile test is most commonly used to determine the flow curve of the materials. However the maximum achievable strain in this test is limited. Furthermore, in an actual deformation process, the state of stress is mostly multiaxial rather than uniaxial. Thus, a simple uniaxial tensile test is of limited use when evaluating the formability of sheet metals. Therefore, a biaxial test, such as bulge test, can predict the metal behavior more close to the reality. Furthermore, the other tests such as Nakazima and Marciniak tests can be used to determine forming limit diagram (FLD) and predict the defect of tearing during the forming process.